Remove Taper from object
Here's my approach showing how you can achieve it using the Cast Modifier (Cylinder) and shrink/flatten tool. 


Add the Cast Modifier and set its mode to Cylinder. Set Factor and Radius values to 1.000. Apply modifier. Next enter the Edit Mode, select all the inner faces with Alt+RMB, then press Alt+S to scale them down. Finally in Object Mode press S,Shift+Z to scale the whole object fitting the original mesh's parameters.